CYA / Acid Demand relationship

[Benji]

Good day all-

Recently installed a SWG and was just reading the fllwng article: Water Balance for SWGs

I don't quite understand this section from it:

qte

Second adjust CYA to between 70 and 80. The biggest mistake that many SWG owners make is NOT having enough CYA in the water! This can create a lot of problems like high acid demand, algae outbreaks, cloudy water, or early cell failure.

/qte

Can someone pls explain how having low CYA can lead to high acid demand?


I appreciate that the real world experience of users in this forum indicate the 30-50 ppm CYA recommended by SWG manufactures is too low, I'm just weary of adding much more than that until I get a feel for things, as I recently drained most of the pool to to drop cya level to 40 from 150 level and it was an ordeal (previous homeowner used pool guy who ran tabs). I am noticing that my acid demand with newly installed SWG seems to have gone up to something like 8-10 oz per day to hold a 7.6 ph even after lowering TA to 60 and adding 50 ppm borates. Or am I just making a fuss and this is typical acid demand?

 

[Flying Tivo]

You dont have to hold ph at 7.6, let it float and see where you pool likes Ph. Even at 8 is ok to swim, you can try to lower TA to 50. Most importan is what TA is your Fill water?

 

[Benji]

You dont have to hold ph at 7.6, let it float and see where you pool likes Ph. Even at 8 is ok to swim, you can try to lower TA to 50. Most importan is what TA is your Fill water?

Why is this important? It is probably about 90-100 considering it was originally at 70ish ta and then after drain and fill, I was back to 90 and have since dropped it to 60.

 

[Rancho Cost-a-Lotta]

Why is this important? It is probably about 90-100 considering it was originally at 70ish ta and then after drain and fill, I was back to 90 and have since dropped it to 60.

Higher levels of TA tend to pull up pH, requiring more MA to control pH. With top offs due to evaporation, your adding higher TA fill water which increases the level in the pool.

Can someone pls explain how having low CYA can lead to high acid demand?

I don't know the answer, but @JoyfulNoise may be able to help.

 

[cowboycasey]

Going from using pucks to SWG is where most people see themselves adding way more MA.. That is because pucks are very acidic and drop the PH and TA all the time... When you go to the SWG you no longer have that constant drop in TA and PH so you have to keep adding MA... With a lower TA and a 7.6 to 7.8 PH the pool may find its sweet spot and only have to add MA once or twice a week.. I have my TA low and my CSI is at the lower -0.30 area with my PH at 7.6 or so.. I might add MA once a month and very little at that..

 

[JoyfulNoise]

Can someone pls explain how having low CYA can lead to high acid demand?

So the article you linked to doesn’t exactly go into detail which is why you are lead to question what CYA could possibly have to do with acid demand. The answer is that it is basically an indirect effect

With an SWG you really want to run it as efficiently as possible to extend its usefulness for as long as possible. What TFP’ers have found is that once the CYA goes below 60ppm, the SWG run time or % output has to start going up to hold the same level of FC. This is true in my pool - I know when it’s time to check and increase my CYA precisely because I see my FC dropping even though my pump run time and % output are the same. It’s a measurable and dramatic drop.

So what would happen if I just let it drop and ran at lower CYA. Well, then I’d have to run my cell a lot more. So why is that problem? Because not only will that use up more of my cell life, it will also cause more aeration (inside the cell) and drive the pH up faster. That is what manifests as an acid demand.

So while the link between the two is indirect, it’s real and will lead to more acid consumption. Is that a bad thing? Not necessarily, it depends on your situation. If you don’t feel comfortable raising your CYA you can keep it below 60ppm. You’ll just likely be using more of your cell lifetime than you have to.

 

[Benji]

So what would happen if I just let it drop and ran at lower CYA. Well, then I’d have to run my cell a lot more. So why is that problem? Because not only will that use up more of my cell life, it will also cause more aeration (inside the cell) and drive the pH up faster. That is what manifests as an acid demand.

This makes perfect sense, thank you for the explanation.

 

[ajw22]

it will also cause more aeration (inside the cell) and drive the pH up faster. That is what manifests as an acid demand.

So while the link between the two is indirect, it’s real and will lead to more acid consumption. Is that a bad thing? Not necessarily, it depends on your situation. If you don’t feel comfortable raising your CYA you can keep it below 60ppm. You’ll just likely be using more of your cell lifetime than you have to.

The Internet never forgets.

Here @JoyfulNoise says...

the aeration caused by the SWG is not a major contributor to pH rise. Yes, the aeration does cause some pH rise but you would be hard pressed to see it without some very specific equipment capable of measuring small changes in pH.

pH rise with SWG

Sorry to belabor, but this has been really bugging me, the seemingly contradictory data from various TFP experts. Chem Geek clearly states "The hydrogen gas bubbles out of the returns and aerates the water and is the main cause of pH rise in SWG pools." I see those very bubbles. They are...

www.troublefreepool.com

And yes you are entitled to change your mind.

But the TFP established dogma has been that SWG's do not contribute to pH rise. Yet it is significant enough to have a mention in Pool School.

My personal observation is I had a more rapid pH rise when I was running an IC40 which required a longer generation % then the IC60 I switched to. And most of my pH rise disappeared for the few weeks when I had a dead cell. And I run a TA of 50-60.

@mknauss consistently says "The SWCG itself is not a significant factor in pH rise." But it is significanct enough to mention in a PS article. And should such statement be qualified by "depending how high your generation % is. With higher SWG generation % the pH rise due to SWG aeration can begin being significant"?

So which is it?

 

[mknauss]

I can only say that my pool pH rise is tied to fill water use. When I put our solar cover on the pool, and still run the SWCG, my pH does not rise. And I am adding very little fill water. When I uncover the pool, SWCG running the same amount, my pH rises, as does my fill water use.

I suppose in those areas that you do not use fill water, or your fill water TA is very low, then the aeration from the SWCG might be more of an effect. But then your overall pH rise will be very small, especially when compared to that in a high volume, high TA fill water area.

 

[ajw22]

I can only say that my pool pH rise is tied to fill water use. When I put our solar cover on the pool, and still run the SWCG, my pH does not rise.

Note that you have a very large cell for your pool size and run a low generation %.

I think the generation % can account for the many different experiences.

I think we need to have a more nuanced explanation for people considering their situations.

 

[JoyfulNoise]

It's not a binary choice, the chemistry is bit more nuanced. The problem, or perhaps the choice, that TFP makes is that "Keep It Simple Stupid" is the dogma and, unfortunately, chemistry is not simple.

There are two sources of pH rise with an SWG - chlorine generation itself (hydroxyl ion formation) AND aeration of water that is over-carbonated. Both of those will raise pH. A lot of the chlorine reactions, oxidation and disinfection, are acidic by nature, that is, they create a proton (H+) as a product of the reaction. So that offsets some of the hydroxyl, OH-, created ... but NOT ALL. Aeration causes permanent pH rise. Therefore an SWG can create residual pH rise and it is not, strictly speaking, pH neutral (in fact, no addition of chlorine to water is pH neutral). How much is dependent on the particulars of the pool in question and the details of the pool water chemistry. Fill water also has a big effect. If you are constantly filling your pool with high TA fill water, and high TA is really anything over 10 or 20 ppm, then there is going to be a pH rise.

Everyone always wants a "simple answer" so they can blame That One Thing™ and sleep better at night ... well, life isn't simple and neither is chemistry. If you want simple, here it is -

If the pH of your pool water is constantly rising, then add acid regularly to reduce it and stop whining about water chemistry maintenance. Then go for a swim, it's really not that hard ...

 

[AUSpool]

I can only say that my pool pH rise is tied to fill water use. When I put our solar cover on the pool, and still run the SWCG, my pH does not rise. And I am adding very little fill water. When I uncover the pool, SWCG running the same amount, my pH rises, as does my fill water use.

Sorry to interject here but that sounds more like your pH rise is tied to your cover by changing the partial pressure potential and top up water consumption.

l think pH rise is tied to all the potential factors which are variable and dependant on each individual pool, as said, if it’s an issue address each factor and add acid if required.

 

[PoolStored]

I think the generation % can account for the many different experiences.

Aren't cells either on or off, it is just the amount of time that they are on?

Running higher % over 6 hours, or lower % (over 24 hours), would not change the amount of time the cell is "on."

 

[ajw22]

Aren't cells either on or off, it is just the amount of time that they are on?

Running higher % over 6 hours, or lower % (over 24 hours), would not change the amount of time the cell is "on."

You are misreading what I wrote.

If we both have the same cell and your pool requires 20% and my pool requires 80%, I will see a more rapid pH rise then you will.

The generation %, due to the longer cell runtime, creates more pH rise.

 

[Benji]

fwiw to the original discussion i also came by this article on the orenda site, basically confirming what @JoyfulNoise said:

Conductivity and Saltwater Pools

Salt chlorine generators need enough conductivity in water to perform correctly. Two factors that impact conductivity are salinity and water temperature.

blog.orendatech.com

Why salt systems raise pH​

Chlorine gas is acidic, and sodium hydroxide is basic. They neutralize each other's pH. All that's left is off-gassing Hydrogen gas, which creates aeration, causing the loss of CO2 (in the form of carbonic acid). That loss of CO2 raises the pH.

This is worth repeating. It's the loss of CO2 in the salt cell (caused by Hydrogen gas bubbling off) that causes the pH to rise in saltwater pools. For years, we–and most of the pool industry–thought it was sodium hydroxide that raised the pH. And it would raise the pH, if not for the acidic chlorine gas (Cl2) that neutralizes it.

And speaking of chlorine gas, one other thing to consider is the length of pipe from the salt cell back to the pool. If it's a short run of pipe, and the return inlets are aimed up at the surface (especially the return closest to the equipment pad), there's a chance that chlorine gas may not have quite enough time to dissolve all the way. You could potentially lose some, which could cause the pH to rise faster.

All is well, however, if you are containing pH with reduced carbonate alkalinity. Because even if your pH is being driven up by the salt system, your pH can only go up to its ceiling. Anything beyond that, and the atmosphere will force CO2 back into the water, keeping your pH at its equilibrium point (ceiling). See the chart below:

So a salt system has a net pH rise due to the loss of CO2, which can be accounted for in our strategy for containing pH using Henry's Law of physics.

In summary, most pool professionals we speak with have a habit of adding more acid than necessary in saltwater pools. We as an industry have been conditioned to think salt cells always raise pH no matter what, and we have to knock down that pH with acid every week. But we're here to tell you that you can limit how high the pH can go by limiting your carbonate alkalinity. Use physics to your advantage.